The present invention relates to a separator plate for a fuel cell. The separator plate comprises at least one discrete fluid distribution feature such as, for example, an open channel formed in a major surface of the separator plate. More particularly, within the thickness of the separator plate, the fluid distribution feature is fluidly isolated from fluid inlets or outlets.
Electrochemical fuel cells convert reactants, namely fuel and oxidants, to generate electric power and reaction products. Electrochemical fuel cells generally employ an electrolyte disposed between two electrodes, namely a cathode and an anode. The electrodes each comprise electrocatalyst disposed at the interface between the electrolyte and the electrodes to induce the desired electrochemical reactions. The location of the electrocatalyst typically defines the electrochemically active area of the electrode. The electrode layers are electrically conductive and fluid permeable, so that the reactant fluids may flow to the electrocatalyst sites from the fuel cell reactant inlet.
The fuel fluid stream which is supplied to the anode may be a gas such as substantially pure hydrogen or a reformate stream comprising hydrogen. Alternatively, a liquid fuel stream such as, for example, aqueous methanol may be used. The oxidant fluid stream, which is supplied to the cathode, typically comprises oxygen, such as substantially pure oxygen, or a dilute oxygen stream such as air.
Solid polymer fuel cells employ a solid polymer electrolyte, or ion exchange membrane. The membrane is typically interposed between two electrode layers, forming a membrane electrode assembly (xe2x80x9cMEAxe2x80x9d). While the membrane is typically proton conductive, it also acts as a barrier, isolating the fuel and oxidant streams from each other on opposite sides of the MEA. The MEA is typically disposed between two plates to form a fuel cell assembly. The plates act as current collectors and provide support for the adjacent electrodes. The assembly is typically compressed to ensure good electrical contact between the plates and the electrodes, in addition to adequate sealing between fuel cell components.
A plurality of fuel cell assemblies may be combined and electrically connected in series or in parallel, to form a fuel cell stack. In a fuel cell stack, a plate may be shared between two adjacent fuel cell assemblies, in which case the plate also serves as a separator to fluidly isolate the fluid streams of the two adjacent fuel cell assemblies.
Fuel cell plates, known as separator plates, may have open channels formed in one or both opposing major surfaces for directing reactants and/or coolant fluids to specific portions of such major surfaces. The open channels also provide passages for the removal of reaction products, depleted reactant streams, and/or heated coolant streams. U.S. Pat. No. 4,988,583 sets forth figures that illustrate an example of a fuel cell separator plate which has an open serpentine channel formed on a major surface of the plate. In this example, the channel is continuous, that is, fluidly connected to a fluid inlet and to a fluid outlet. U.S. Pat. No. 5,300,370 (xe2x80x9cthe ""370 patentxe2x80x9d) discloses several fluid channel configurations formed in a major surface of fuel cell separator or flow field plates. For example, FIG. 8 of the ""370 patent illustrates a plurality of parallel continuous channels which are each fluidly connected to an inlet and an outlet. FIG. 9 of the ""370 patent illustrates a flow field plate that employs discontinuous flow field channels, but each of these channels is fluidly connected by an open-faced channel to one of the fluid inlet or the fluid outlet.
One approach to improving fuel cell performance is to increase the open channel area to increase contact between the reactant streams flowing in the channels and the adjacent electrochemically active area of the adjacent electrode. Performance is improved by increasing the accessibility of the reactant fluids to the electrocatalyst at the interface between the electrolyte and the electrode. A purpose of the open-faced flow field channels formed in the separator plates is to direct the reactant fluids across substantially the whole of the electrochemically active area. These open-faced channels are defined by adjoining xe2x80x9cland areasxe2x80x9d that contact and support the electrode layer, thereby substantially preventing the electrode from deflecting into the open channel. However, reactant fluids will generally travel through the fuel cell from an inlet to an outlet along the path that offers the lowest pressure loss. The pressure loss sustained when the reactant fluid travels along the channel is typically less than the pressure loss that would be sustained if the reactant fluid were to flow through the fluid permeable electrode. Therefore, when a continuous channel is provided between a fuel cell inlet and a fuel cell outlet, the majority of the reactant fluid stream will travel within the channel and diffuse into the electrode areas mostly in the regions next to the open-faced channels. A problem with conventional flow field arrangements is that reactant fluids typically only have limited access to the electrode areas adjacent the land areas by diffusion, and some degree of xe2x80x9cflowxe2x80x9d if there is a consistent pressure differential between channels on opposite sides of a land area. For this reason, conventional flow field plates tend to be designed to increase the open channel area and reduce the land areas.
However, because the land areas are needed to support the electrode and provide an electrically conductive path from the electrodes to the separator plates for current collection, a compromise must be made between reducing the size of the land areas to increase direct exposure of the electrodes to the reactant fluid flowing in the channel, and providing sufficiently sized land areas for adequately supporting the electrode layer and providing adequate electrical conductance for current collection.
Since there may be little or no fluid flow to the electrode layer where it contacts the separator plate land areas, water may accumulate within the electrode layer in these areas. The accumulation of water may compound the problem by flooding the electrode in the portions adjacent the land areas, thus making the flooded areas even less accessible to the reactant fluids.
An electrically conductive, substantially fluid impermeable separator plate for an electrochemical cell comprises:
(a) a substantially planar major surface for facing a fluid permeable electrode of the electrochemical cell;
(b) a fluid inlet through which a fluid may be directed to the planar major surface;
(c) a fluid outlet through which fluid may be removed from the planar major surface;
(d) at least one discrete fluid distribution feature formed in the planar major surface wherein, within the thickness of the plate, the fluid distribution feature is fluidly isolated from the fluid inlet and the fluid outlet.
In a preferred embodiment, the electrochemical cell is a solid polymer fuel cell.
The discrete fluid distribution feature is preferably a channel that is oriented substantially perpendicular to the direction of fluid flow to and from the discrete distribution channel. That is, unlike conventional flow field channels, where the majority of the fluid travels in the same direction as the channel orientation (along the length of the channel), in preferred embodiments, fluid flowing to/from the discrete fluid distribution channel enters or exits the discrete fluid distribution channel along a flow path that is substantially perpendicular to the longitudinal orientation of the discrete fluid distribution channel. In the present flow field fluid distribution areas where there are discrete fluid distribution channels, in addition to fluid flow along the discrete fluid distribution channel, the fluid is directed across the land areas between adjacent discrete fluid distribution channels and/or adjacent inlet or outlet channels.
For a separator plate that comprises a plurality of discrete fluid distribution channels that are oriented substantially parallel to one another, the fluid preferably flows between adjacent discrete fluid distribution channels in a direction that is perpendicular to the discrete fluid distribution channels.
In embodiments that further comprise inlet or outlet channels extending from, and in fluid communication, with respective inlet or outlet openings or manifolds, the respective inlet or outlet channels are preferably substantially parallel with the at least one discrete fluid distribution channel, and fluid flows between the discrete fluid distribution channel and the inlet or outlet channel in a direction that is substantially perpendicular to the discrete fluid distribution channel and the inlet and/or outlet channels.
In preferred embodiments the discrete fluid distribution channel is oriented substantially perpendicular to a straight line drawn between the fluid inlet and the fluid outlet. In other preferred embodiments the discrete fluid distribution channel is oriented substantially parallel to a straight line drawn between the fluid inlet and the fluid outlet.
The substantially planar major surface of the separator plate preferably comprises a fluid distribution area that corresponds to and is coextensive with the electrochemically active area of the adjacent electrode when the separator plate is part of a fuel cell assembly. The discrete fluid distribution channel is preferably a substantially straight channel that extends substantially between opposing edges of the fluid distribution area. In one preferred embodiment the fluid distribution area is substantially in the shape of a rectangle and the discrete fluid distribution channel is substantially aligned with the longitudinal axis of the fluid distribution area.
A preferred embodiment of the separator plate comprises a hybrid fluid distribution area that comprises one or more serpentine channels and at least one discrete straight fluid distribution channel. For example, the major planar surface comprises at least one serpentine fluid channel that extends from at least one of the fluid inlet and the fluid outlet. The serpentine fluid channel extends over a portion of the planar major surface, while at least one discrete fluid distribution channel extends over another portion of the fluid distribution area.
The separator plate preferably further comprises openings penetrating the plate for forming fluid manifolds when the plate is one of a plurality of plates placed one on top of the other to form a fuel cell stack. The openings are aligned in adjacent plates, and are typically fluidly connected to fluid passages formed in the fuel cell stack end plate assemblies.
The features formed in the major planar surface of the separator plate may be formed by embossing a material comprising expanded graphite. More particularly, when the plate material is a formable material like expanded graphite, any of the plate features, which may include, for example, one or more discrete fluid distribution channels, inlet channel(s), outlet channel(s), grooves for receiving seals, and fluid manifold openings, may be formed by embossing methods.
In another embodiment, the separator plate may be made from a molded composite material comprising carbon. The carbon component provides electrical conductivity and is compatible with the operating environment and the fluids that flow through conventional solid polymer electrolyte fuel cells. As an alternative to using a molding process to form the discrete fluid distribution channel, a die cutter or mill may be employed to cut the discrete channel into the major planar surface of the plate.
The separator plate may be impregnated with resin to improve the desired mechanical properties such as durability, impermeability, and hydrophobicity. The plate is preferably hydrophobic to facilitate the flow of water within the fluid distribution area. It is important to facilitate water flow within the fluid distribution area because water is needed to keep the electrolytic membrane moist to increase its ion conductivity. It is equally important for excess water to be removed from the fluid distribution area to prevent flooding which might prevent reactants from accessing the electrochemically active areas of the electrode.
In a preferred embodiment, the discrete fluid distribution channel has a substantially uniform cross-sectional area along its length. In essence, each of the discrete fluid distribution channels acts as a fluid header for distributing reactant fluid along the length of the discrete channel. A uniform channel cross-section promotes a more even distribution of the reactant along the length of the discrete channel.
The present separator plate is preferably employed in an electrochemical fuel cell stack that comprises:
(a) a plurality of membrane electrode assemblies comprising a pair of fluid permeable electrodes, wherein each one of the membrane electrode assemblies is interposed between planar major surfaces of two substantially fluid impermeable separator plates; and
(b) at least one discrete fluid distribution feature (preferably a discrete channel) formed in one of the planar major surface of at least one of the two plates, wherein within the thickness of the respective one of the separator plates, the fluid distribution channel is fluidly isolated such that it is not connected to a fluid inlet or a fluid outlet by any fluid conduits within the thickness of the respective one of the separator plates.
The at least one discrete fluid distribution channel is preferably one of a plurality of discrete fluid distribution channels which are each fluidly isolated within the thickness of the respective one of the flow field plates. The plurality of discrete fluid distribution channels are preferably substantially parallel to one another, so that the reactant fluid flowing across the fluid distribution area is distributed across substantially the entire area.